Mei Zhangb,
Jin-Kui Ouyanga,
Qiao-Lin Xu*c,
Shao-Bo Liua,
Tao Qiana,
Li-Mei Dong‡
a and
Jian-Wen Tan*a
aState Key Laboratory for Conservation and Utilization of Subtropical Agro-Bioresources, Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, China. E-mail: jwtan@scau.edu.cn; Tel: +86-20-85280256
bBeijing Center for Physical and Chemical Analysis, Beijing 100089, China
cGuangdong Provincial Key Laboratory of Silviculture, Protection and Utilization, Guangdong Academy of Forestry, Guangzhou 510520, China. E-mail: qlxu@sinogaf.cn
First published on 2nd February 2021
Three new thymol derivatives, 7-formyl-9-isobutyryloxy-8-hydroxythymol (1), 7,9-di-isobutyryloxy-8,10-dehydrothymol (2) and 2α-methoxyl-3β-methyl-6-methylol-2,3-dihydrobenzofuran (3), along with five known ones (4–8), were isolated from the aerial parts of the invasive plant Ageratina adenophora. Their structures were elucidated by extensive spectroscopic analysis and they were all isolated from the aerial part of A. adenophora for the first time. These compounds, except 8, selectively showed in vitro antimicrobial activity against three Gram-(+) and two Gram-(−) bacterial strains. In particular, compounds 1 and 5 showed notable in vitro antimicrobial activity against all five bacterial strains with IC50 values ranging from 3.9 to 15.6 μg mL−1, as compared to reference compound kanamycin sulfate with a MIC value 1.9–3.9 μg mL−1. Compounds 1 and 5 were further revealed to show in vitro cytotoxic activity against three tested human tumor (MCF-7, NCI-H460 and HeLa) cell lines, with IC50 values ranging from 7.45 to 28.63 μM. Compounds 7 and 8 selectively showed slight but detectable in vitro cytotoxicity toward MCF-7 and NCI-H460 cell lines, with IC50 values 44.65–83.19 μM. No cytotoxic effects were detected in the bioassay of the other four thymol derivatives. The present results provide new data to support that the aerial parts of A. adenophora are a rich source of bioactive chemicals valuable in medicinal applications.
In nature, A. adenophora is seldom attacked by microorganisms and insects. This suggests a rich defense related to bioactive chemicals, that possibly might also be pharmaceutically valuable, would exist in this plant. Previously, some literature reported that this plant was used in Nigeria and India as traditional or folk herb medicine for treatment of many human diseases, like fever, diabetes, inflammation, etc.6,7 To date, some terpenoids, flavonoids, phenylpropanoids, coumarins, sterols and alkaloids have been reported from this plant,8–10 with part of them exhibiting phytotoxic,11 allelopathic,12,13 antifungal14 and antifeedant15 activities. In our recent study, some bioactive compounds, including phytotoxic phenolics,16 antifungal monoterpenes17 and antibacterial quinic acid derivatives,18 were also revealed from the roots or aerial parts of this plant. In continuation of the work to clarify those potentially new and bioactive chemicals in the aerial parts of A. adenophora, eight thymol derivatives including three new (1–3) and five known (4–8) ones are here further obtained (Fig. 1). We herein report the isolation and structural elucidation of these compounds, as well as describe their in vitro antimicrobial activity against five bacteria and cytotoxicity toward three human cancer cell lines.
Compound 1 was isolated as a yellow oil and determined to have a molecular formula C14H18O5 on the basis of HR-ESI-MS data (m/z 289.1056 [M + Na]+, calcd. for C14H18O5Na, 289.1052). The IR absorptions at 3424 and 1695 cm−1 revealed the presence of hydroxyl and carbonyl groups in the molecule. The existence of three methyl groups was revealed by 1H NMR spectrum which provided methyl signals at δH 1.66 (3H, s), 1.12 (3H, s) and 1.13 (3H, s). The three aromatic proton signals at δH 7.34 (1H, d, J = 1.2 Hz), 7.18 (1H, d, J = 8.4 Hz), and 7.35 (1H, dd, J = 8.4, 1.2 Hz) were indicative of a typical pattern of 1,3,4-trisubstituted phenyl group.22 The 13C NMR (DEPT) spectra (Table 2), coupled with HSQC spectral analysis, revealed the presence of a formyl group at δC 191.9 (C-7), a carbonyl group at δC 177.7 (C-1′′), an oxymethylene group at δC 70.4 (C-9), a methine group at δC 33.9 (C-2′′), and an oxygenated quaternary carbon at δC 77.7 (C-8). Taken together these spectral data and the molecular formula into consideration, the existence of two hydroxyl groups could further be deduced. In the 1H–1H COSY spectrum, significant correlation signals for two proton systems were displayed, one at C-5 through C-6 and the other at C-3′′ through C-4′′ (Fig. 2), corresponding to structure fragments of –CH(5)–CH(6)– and CH3(3′′)–CH(2′′)–CH3(4′′), respectively. In the HMBC spectrum, the exhibition of correlation signals of H-7 (δH 9.92) with C-2 and C-6, and of H-2, H-6 with C-7 (δC 191.9) evidenced the connection of C-7 with C-1. The HMBC correlations of Me-10, H-9 with C-4 supported the connection of C-8 with C-4. The HMBC correlations of Me-3′′, Me-4′′ with C-1′′ supported the connection of C-1′′ with C-2′′. The HMBC correlations of H-9 with C-1′′ supported the ester bond linkage of C-1′′ with C-9. Further consideration of the substitution pattern of the aromatic ring and the other NMR data led to the assignment of the two hydroxyl groups at C-3 and C-8, respectively. Based on these above spectroscopic analyses, we can unambiguously establish the planner structure of 1 as depicted in Fig. 1. However, it is presently difficult to determine the configuration of C-8. Thus, the structure of 1 was elucidated as 7-formyl-9-isobutyryloxy-8-hydroxythymol.
Compound 2 was assigned the molecular formula C18H24O5, as deduced from ESI-MS and HR-ESI-MS data. Comparative analysis of the spectroscopic data and literature precedents21,23,24 supported 2 to be also a thymol derivative. Careful comparison of 1H and 13C NMR data (Tables 1 and 2) of 2 with those of known compound 8 revealed the major differences of the two compounds that the resonances for Me-10, Me-11 and the sp3 quaternary carbon C-8 in compound 8 were replaced by signals [δH 5.28 (1H, Ha-10), 5.47 (1H, Hb-10); δC 141.6 (C-8), 116.8 (C-10)] for a terminal double bond in 2. In the HMBC spectrum, significant correlation signals of H2-10 to C-4 and C-9 were observed, which confirmed the location of the terminal double bond between C-8 and C-10. The 1H–1H COSY spectrum and HMBC correlations (Fig. 2) of Me-3′ and Me-4′ with C-1′, of Me-3′′ and Me-4′′ with C-1′′, of H-7 with C-1′, and of H-9 with C-1′′, evidenced the ester bond linkages of C-7 and C-9 with an individual isobutyryl group, respectively. The presence of diagnostic proton signals at δH 6.92 (1H, d, J = 1.2 Hz), 7.07 (1H, d, J = 7.6 Hz), and 6.84 (1H, dd, J = 7.6, 1.2 Hz) supported the appearance of the core 1,3,4-trisubstituted phenyl moiety. The HMBC correlations of H-2 and H-6 with C-7, of H-10 and H-9 with C-4, and of H-5 with C-8 confirmed the connections of C7 with C-1, C-4 with C-8, and the location of a hydroxyl group at C-3. Therefore, compound 2 was determined as 7,9-di-isobutyryloxy-8,10-dehydrothymol.
Position | 1a | 2a | 3b |
---|---|---|---|
a Recorded at 600 MHz in CDCl3.b Recorded at 400 MHz in CD3OD. | |||
2 | 7.34 (1H, d, 1.2) | 6.92 (1H, d, 1.2) | 5.21 (1H, d, 2.0) |
3 | — | — | 3.15 (1H, brq, 7.2) |
4 | — | — | 7.12 (1H, d, 7.6) |
5 | 7.18 (1H, d, 8.4) | 7.07 (1H, d, 7.6) | 6.87 (1H, brd, 7.6) |
6 | 7.35 (1H, dd, 8.4, 1.2) | 6.84 (1H, dd, 7.6, 1.2) | — |
7 | 9.92 (1H, s) | 5.06 (2H, s) | 6.79 (1H, brs) |
9 | 4.47 (1H, d, 12.0) | 4.73 (2H, s) | — |
4.33 (1H, d, 12.0) | |||
10 | 1.66 (3H, s) | 5.47 (1H, d, 1.2) | 1.23 (3H, d, 7.2) |
5.28 (1H, d, 1.2) | |||
11 | — | — | 4.53 (2H, s) |
12 | — | — | 3.47 (3H, s) |
2′ | — | 2.62 (1H, sept, 7.2) | — |
3′ | — | 1.20 (3H, d, 7.2) | — |
4′ | — | 1.20 (3H, d, 7.2) | — |
2′′ | 2.57 (1H, sept, 7.2) | 2.62 (1H, sept, 7.2) | — |
3′′ | 1.12 (3H, d, 7.2) | 1.20 (3H, d, 7.2) | — |
4′′ | 1.13 (3H, d, 7.2) | 1.20 (3H, d, 7.2) | — |
Position | 1a | 2a | 3b |
---|---|---|---|
a Recorded at 150 MHz in CDCl3.b Recorded at 100 MHz in CD3OD. | |||
1 | 137.4 (C) | 138.2 (C) | — |
2 | 118.9 (CH) | 115.5 (CH) | 115.5 (CH) |
3 | 157.1 (C) | 153.7 (C) | 44.4 (CH) |
4 | 132.5 (C) | 125.1 (C) | 124.7 (CH) |
5 | 127.0 (CH) | 129.6 (CH) | 120.8 (CH) |
6 | 120.5 (CH) | 119.3 (CH) | 143.3 (C) |
7 | 191.9 (CH) | 65.5 (CH2) | 109.3 (CH) |
8 | 77.7 (C) | 141.9 (C) | 159.3 (C) |
9 | 70.4 (CH2) | 65.5 (CH2) | 131.8 (C) |
10 | 25.8 (CH3) | 116.8 (CH2) | 18.9 (CH3) |
11 | — | — | 65.2 (CH2) |
12 | — | — | 56.2 (CH3) |
1′ | — | 176.9 (C) | — |
2′ | — | 34.0 (CH) | — |
3′ | — | 18.8 (CH3) | — |
4′ | — | 18.8 (CH3) | — |
1′′ | 177.7 (C) | 177.8 (C) | — |
2′′ | 33.9 (CH) | 34.0 (CH) | — |
3′′ | 18.8 (CH3) | 18.9 (CH3) | — |
4′′ | 18.8 (CH3) | 18.8 (CH3) | — |
The molecular formula of compound 3 was determined as C11H14O3 by the HR-EI-MS, due to a quasi-molecular ion peak at m/z 194.0938 (calcd. for C11H14O3, 194.0937). The IR absorption at 3448 cm−1 revealed the presence of hydroxyl group in the molecule. The 1H and 13C (DEPT) NMR spectra displayed closely related signals with those of 2α-methoxyl-3β-methyl-6-(acetyl-O-methyl)-2,3-dihydrobenzofuran,19 a known thymol compound which was also obtained as compound 4 in the present study. Careful comparison of their NMR spectral data indicated the major difference that the spectroscopic resonances for the acetoxy group connected to C-11 in 4 were absent in 3, suggesting that the acetoxy group located at C-11 in 4 was replaced by a free hydroxyl group in 3. Accordingly, we can preliminarily establish the structure of compound 3 as shown in Fig. 1, and this deduction was further well supported by 1H–1H COSY and HMBC analysis (Fig. 2). Furthermore, the presented coupling constant of H-2 (J2,3 = 2.0 Hz) and the NOE correlation between H-2 and H3-10 in the NOESY spectrum supported the β-orientation of Me-10 and the α-orientation of the methoxy group at C-2.19,25 Therefore, compound 3 was assigned as 2α-methoxyl-3β-methyl-6-methylol-2,3-dihydrobenzofuran.
Among these thymol derivatives, 1–3 are new compounds that are here reported from nature for the first time. Compound 3 is structurally characterized with a dihydrobenzofuran skeleton and this type of monoterpene is rare in natural products.25 Compound 5 was previously only reported from plant Eupatorium fortunei and this is the first time for it being isolated from A. adenophora.20 The other four known compounds 4, 6, 7 and 8 are here also isolated and identified from the aerial parts of A. adenophora for the first time.
Aimed to explore their potential and undiscovered pharmacological activity of these isolated thymol derivatives, compounds 1–8 were evaluated for their in vitro antimicrobial activity by testing their MIC values, using a bioassay method as previously we used and described.26,27 Table 3 lists the results obtained for these compounds on the viability of five tested bacterial strains, compared to kanamycin sulfate (KS) as a reference compound. Among them, compounds 1 and 5 were found to be strongly active against all the five assayed microorganisms with MIC values ranging from 3.9 to 15.6 μg mL−1, which were comparable to the positive control KS (MICs = 1.9 to 3.9 μg mL−1). Compounds 2, 3, 4, 6 and 7 only selectively showed antibacterial activity (MIC 31.3–62.5 μg mL−1) against three tested Gram-(+) bacteria, i.e. S. aureus, B. cereus and B. thuringiensis. While no antibacterial activity was detected for compound 8 toward all the five tested bacterial strains. From the result, it seems to show that the existence of both the hydroxyl group at C-3 and C-8 would be important for this group of thymol compounds to display their antibacterial potentials.
Compounds | Staphylococcus aureus | Bacillus cereus | Bacillus thuringiensis | Escherichia coli | Salmonella enterica |
---|---|---|---|---|---|
a KS = kanamycin sulfate. | |||||
1 | 7.8 | 3.9 | 7.8 | 15.6 | 15.6 |
2 | >100 | 62.5 | >100 | >100 | >100 |
3 | 31.3 | 31.3 | 62.5 | >100 | >100 |
4 | 31.3 | 62.5 | 62.5 | >100 | >100 |
5 | 7.8 | 7.8 | 15.6 | 15.6 | 15.6 |
6 | 62.5 | 62.5 | 62.5 | >100 | >100 |
7 | >100 | 62.5 | 62.5 | >100 | >100 |
8 | >100 | >100 | >100 | >100 | >100 |
KS | 1.9 | 3.9 | 3.9 | 3.9 | 3.9 |
Compounds 1–8 were further tested for their in vitro cytotoxicity against human cancer MCF-7, HeLa and NCI-H460 cell lines, using a microdilution titre technique as recently we described.28 The resulting IC50 values are displayed in Table 4, compared to Adriamycin as positive control. Compounds 1 and 5 were found to show strong or moderate cytotoxicity against all the three tested cancer cell lines, with IC50 values ranging from 7.45 to 28.63 μM. Compounds 7 and 8 showed slight but detectable cytotoxicity toward MCF-7 and NCI-H460 cell lines, with IC50 values ranging from 44.65 to 83.19 μM. While, no obvious cytotoxic activity was detected for the other compounds in this bioassay. Comparison of the chemical structures and the cytotoxic activity of these compounds indicated that the existence of both the hydroxyl group at C-3 and C-8 would be important for this group of thymol derivatives to fully display their cytotoxic potentials.
Compounds | MCF-7 | NCI-H460 | HeLa |
---|---|---|---|
a Values represent mean ± SD (n = 3) based on three individual experiments. | |||
1 | 7.45 ± 0.22 | 8.32 ± 0.21 | 9.45 ± 0.46 |
2–4 | >100 | >100 | >100 |
5 | 11.54 ± 0.86 | 15.67 ± 1.03 | 28.63 ± 1.93 |
6 | >100 | >100 | >100 |
7 | 44.65 ± 4.08 | 52.74 ± 5.16 | >100 |
8 | 83.19 ± 6.55 | 77.42 ± 5.06 | >100 |
Adriamycin | 0.78 ± 0.06 | 1.12 ± 0.05 | 0.54 ± 0.04 |
As a well-known invasive plant, A. adenophora has attracted much attention of scientists to investigate its invasion mechanisms. Since that this plant is accumulating a huge biomass at its invasion areas, to explore the potential utilization of A. Adenophora is gradually also concentrated and emphasized by some researchers. Up to date, phytochemical studies have indicated that structurally diverse chemicals exist in this invasive species. Our present findings further support that the aerial part tissue of this plant is rich in bioactive natural products valuable to be explored for medicinal usage. It is interesting to note that compound 5 was reported as a natural product capable of strongly inhibiting the growth of Microcystis aeruginosa,20 suggesting that these thymol derivatives, at least for compound 5, might have some allelopathic potential to contribute the invasion success of A. adenophora. Noteworthily, for all these thymol derivatives identified from A. adenophora, the carbon C-7 (or C-11 in compounds 3 and 4) is generally appeared as an oxygenated carbon. While for those thymol compounds reported from some other Eupatorium plants, such as thymol compounds from E. fortunei and E. cannabinum,22–24 usually exhibited at C-7 is a methyl group (an unoxygenated carbon). Thus, the general oxygenation extent of thymol compounds at C-7 might have some chemotaxonomic significance for plants in Adenophora and (or) Eupatorium genus. Furthermore, it is evident that compounds 3 and 4 contain a typical dihydrobenzofuran skeleton. Dihydrobenzofuran type monoterpene is rare in nature and the discovery of dihydrobenzofuran type new compound 3 suggests that some so far undiscovered rare monoterpenes would still exist in the aerial parts of A. adenophora worthy of further investigation.
The petroleum ether fraction (93.1 g) was subjected to silica gel CC (1000 mm × 120 mm i.d.), eluting with a gradient of CHCl3–MeOH (100:
0 to 90
:
10, v/v) to give twelve fractions (P1–P12) after pooled according to their TLC profiles. Fraction P6 (20.6 g), obtained by elution with CHCl3–MeOH (98
:
2, v/v), was applied to silica gel CC (800 × 75 mm i.d.) eluted with a gradient of petroleum ether–acetone (100
:
0 to 80
:
20, v/v) to obtain nine fractions (P6-1–P6-9). Fraction P6-4 (7.4 g) was subjected to silica gel CC (800 × 50 mm i.d.) eluted with petroleum–acetone (300
:
1 to 100
:
5) in a gradient to obtain sub-fractions P6-4-4–P6-4-9, of which sub-fraction P6-4-4 (600.0 mg) was separated by preparative HPLC (flow rate 8 mL min−1) using MeOH–H2O (70
:
30, v/v) to afford compound 6 (tR = 37.5 min, 2.4 mg). Fraction P6-7 (6.2 g) was applied to an ODS CC using MeOH–H2O (30
:
70 to 90
:
10) to obtain sub-fractions P6-7-1–P6-7-12. Subsequently sub-fraction P6-7-8 (19.2 mg) was purified by Sephadex LH-20 CC (1500 mm × 25 mm i.d.) eluted with CHCl3–MeOH (20
:
80, v/v) to afford compounds 2 (2.1 mg) and 8 (2.5 mg), and sub-fraction P6-7-3 (930.0 mg) was purified by a Sephadex LH-20 CC using acetone to afford compound 7 (25.1 mg). Fraction P6-8 (340.0 mg) was subjected to Sephadex LH-20 CC and preparative HPLC (flow rate 8 mL min−1) using MeOH–H2O (45
:
55 to 50
:
50, v/v) as mobile phase to afford compound 1 (tR = 48.0 min, 6.7 mg). Fraction P6-9 (600.0 mg) was subjected to silica gel CC eluted with petroleum–acetone (100
:
1 to 90
:
10, v/v) in gradient to obtain sub-fractions P6-8-1 and P6-8-2, and sub-fraction P6-8-2 (45.5 mg) was further purified by an ODS CC using MeOH–H2O (65
:
35, v/v) as eluent to afford compound 4 (3.0 mg).
The EtOAc-soluble fraction (80.0 g) was subjected to silica gel CC (1000 mm × 120 mm i.d.) using a gradient of CHCl3–MeOH (95:
5 to 60
:
40, v/v) to give ten fractions (E1–E10). Fraction E6 (12.0 g), obtained by elution with CHCl3–MeOH (85
:
15, v/v), was subjected to silica gel CC eluted with CHCl3–MeOH (40
:
1 to 9
:
1, v/v) in gradient to obtain sub-fractions E6-1–E6-8. Subfraction E6-2 (213.1 mg) was further applied to ODS CC eluted with MeOH–H2O (20
:
80 to 90
:
10, v/v) to afford compound 3 (7.0 mg). Fraction E6-5 (2.7 g) was subjected to ODS CC using MeOH–H2O (10
:
90 to 70
:
30, v/v) to afford five sub-fractions (E6-5-1–E6-5-5), of which sub-fraction E6-5-1 (312.0 mg) was further purified by Sephadex LH-20 CC using MeOH to afford compound 5 (30.0 mg).
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra08885d |
‡ Presently work at Guangdong Eco-Engineering Polytechnic, Guangzhou 510520, China. |
This journal is © The Royal Society of Chemistry 2021 |